The present invention relates generally to radiation sensitive photoresist compositions and particularly to compositions containing aqueous alkali soluble resins together with naphthoquinone diazide sensitizing agents.
It is well known in the art to produce positive photoresist formulations such as those described in U.S. Pat. Nos. 3,666,473, 4,115,128, 4,173,470 and 4,550,069. These include alkali soluble phenol-formaldehyde novolak resins together with light sensitive materials, usually a substituted naphthoquinone diazide compound. The resins and sensitizers are dissolved in an organic solvent or mixture of solvents and are applied as a thin film or coating to a substrate suitable for the particular application desired.
The resin component of these photoresist formulations is soluble in aqueous alkaline solutions, but the naphthoquinone sensitizer acts as a dissolution rate inhibitor with respect to the resin. Upon exposure of selected areas of the coated substrate to actinic radiation, however, the sensitizer undergoes a radiation induced structural transformation and the exposed areas of the coating are rendered more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is immersed in an alkaline developing solution while the unexposed areas are largely unaffected, thus producing a positive relief pattern on the substrate.
In most instances, the exposed and developed substrate will be subjected to treatment by a substrate-etchant solution. The photoresist coating protects the coated areas of the substrate from the etchant and thus the etchant is only able to etch the uncoated areas of the substrate, which in the case of a positive photoresist, correspond to the areas that were exposed to actinic radiation. Thus, an etched pattern can be created on the substrate which corresponds to the pattern on the mask, stencil, template, etc., that was used to create selective exposure patterns on the coated substrate prior to development.
The relief pattern of the photoresist on substrate produced by the method described above is useful for various applications including as an exposure mask or a pattern such as is employed in the manufacture of miniaturized integrated electronic components.
The properties of a photoresist composition which are important in commercial practice include the photospeed of the resist, development contrast, resist resolution, and resist adhesion.
Resist resolution refers to the capacity of a resist system to reproduce the smallest equally spaced line pairs and intervening spaces of a mask which is utilized during exposure with a high degree of image edge acuity in the developed exposed spaces. In many industrial applications, particularly in the manufacture of miniaturized electronic components, a photoresist is required to provide a high degree of resolution for very small line and space widths (on the order of one micron or less).
The ability of a resist to reproduce very small dimension, on the order of a micron or less, is extremely important in the production of large scale integrated circuits on silicon chips and similar components. Circuit density on such a chip can only be increased, assuming photolithography techniques are utilized, by increasing the resolution capabilities of the resist.
Photoresists are generally categorized as being either positive working or negative working. In a negative working resist composition, the imagewise light struck areas harden and form the image areas of the resist after removal of the unexposed areas with a developer. In a positive working resist the exposed areas are the non-image areas. The light struck parts are rendered soluble in aqueous alkali developers. While negative resists are the most widely used for industrial production of printed circuit boards, positive resists are capable of much finer resolution and smaller imaging geometries. Hence positive resists are the choice for the manufacture of densely packed integrated circuits.
In many commercial application, it is desirable to convert a high resolution quinone diazide type positive resist for a negative working application. There is interest in the field of image reversal because of the utility of this process in practical device manufacturing. Among the practical aspects of image reversal are the elimination of the need for a dual set of complementary masks to do both positive and negative imaging, greater resolution and process latitude than with positive imaging alone, reduction in standing wave effects, and higher thermal stability. In this regard, several methods have been suggested for such image reversal. See for example: "Image Reversal: The Production of a Negative Image in a Positive Photoresist" by S. A. MacDonald et al. p. 114, IBM Research Disclosure, 1982; "Image Reversal of Positive Photoresist". A New Tool for Advancing Integrated Circuit Fabrication" by E. Alling et al., Journal of the Society of Photo-Imaging Engineers, Vol. 539, p.194, 1985; M. V. Buzuev et.al. "Producing a Negative Image on a Positive Photoresist" SU 1,109,708; German Patent DE 52 9054, C2, 1975, Assigned to H. Moritz and G. Paal, Making a Negative Image; and U.S. Pat. No. 4,104,070.
Each of these disclosures suffer from several drawbacks. A major disadvantage of current image reversal processes is the need for an additional processing step which involves treatment with either salt forming compounds or high energy exposure sources such as electron beams. The present invention provides a mechanism which involves the formation of a catalytic amount of a photogenerated acid which cross links the resin in the exposed region.
The invention provides a unique chemical composition, which when processed in a slightly modified manner to the usual and customary method of lithographic processing, yields a totally unexpected negative, reversed tone image from an otherwise expected positive type photosensitizer.
Among the advantages realized by this highly desirable result are improvement in the relationship between exposure energy and resulting line width, improved process latitude, improvement in developed image resolution, substantial elimination of reflective notching, enhanced photosensitivity, improved thermal stability of the resulting image, improved adhesion between the photoresist and commonly used substrates, and superior storage stability and shelf life of the photoresist.